The invention relates to a method for controlling the quantity of air in a self-contained air supply system for a chassis. Such air supply systems are used, for example, for controlling the required level of motor vehicles.
Such an air supply system is known from EP 1 243 447 A2. According to its FIG. 1, this air supply system is composed essentially of an air supply unit and a plurality of air springs for supporting the vehicle chassis. The air supply unit mainly includes a compressor and an air accumulator. The compressor is connected here on the intake side to the atmosphere and on the pressure side to the air accumulator via an air drier and a first 2/2 way valve. The compressor thus supplies the air accumulator with fresh air from the atmosphere. The compressor is connected to the air springs on the intake side via a second 2/2 way valve. As a result, the compressor transfers compressed air from the air springs into the air accumulator via the first 2/2 way valve. The compressor is also connected on the intake side to the air accumulator via a third 2/2 way valve, and is also connected on the pressure side to the air springs via a fourth 2/2 way valve. Compressed air is thus fed from the air accumulator into the air springs. The air springs are arranged parallel to one another, each air spring being assigned a 2/2 way valve and all the 2/2 way valve units being connected to the air supply unit via a collecting line. A pressure sensor is located in the common collecting line of the directional valve unit. Such self-contained air supply systems operate within a previously defined power range whose limits are often undershot as a result of the fact that a quantity of compressed air escapes as a result of a leakage or the limits of said air supply systems are often exceeded as a result of the fact that the quantity of compressed air is increased as a result of a rise in temperature. Within the ride level control of the vehicle this has the effect of slowing down the raising of the vehicle body if the quantity of compressed air is too low and of slowing down the lowering of the vehicle body if the quantity of compressed air is too high. In order to ensure the power range is in its admissible limits, a sufficient quantity of compressed air must therefore always be present in the air supply system. For this purpose, the pressure in the air springs and in the air accumulator is continuously measured using the pressure sensor and the excess of or the demand for an additional quantity of compressed air is calculated therefrom. When there is an excess, a quantity of compressed air is let out of the air supply system, and when there is a demand for a quantity of compressed air fresh air is added to the air accumulator. This aforesaid method satisfies the technical requirements. However, the expenditure in terms of equipment is relatively high. For example, it is necessary to use a pressure sensor with corresponding cabling. This entails additional costs. Furthermore, the pressure sensor with its cabling requires considerable installation space which is generally not available in vehicle engineering and which therefore leads to compromises in the implementation of the air supply system. This also entails higher costs.
DE 101 22 567 C1 discloses a further method for controlling the quantity of air in which the influence of the instantaneous load on the control of the quantity of air is excluded and control of the quantity of air is therefore carried out only when there is a deviation from the layout range of air quantity due to leakage or temperature fluctuation. In this context, the instantaneous quantities of compressed air in the air accumulator and in the air springs are determined by measuring the pressures using a pressure sensor and multiplying them by the known volume of the air accumulator and by the volume of the air springs determined by a travel measurement. This quantity of compressed air which is determined in this way for the air supply system is compared with the optimum quantity of compressed air for a layout load. If the quantity of compressed air which is determined is smaller than a minimum necessary quantity of compressed air, a specific quantity of compressed air must be added, and if it is larger than a maximum admissible quantity of compressed air a specific quantity of compressed air must be let out. The time necessary to add air or let air out is determined from a known controlling speed compressed air quantity characteristic curve and the corresponding valves or the compressor in the air supply system are activated for this time period. This method also requires the presence of a pressure sensor with all its disadvantages which have already been described. In addition, this method is relatively complex in terms of software technology because the actual volume quantity always has to be calculated in order to determine the time necessary to add or let out air.
The object is therefore to simplify the method of the generic type for controlling the quantity of air in a closed air supply system.